The Envelope

The most important thing about a Passive House is the envelope: the foundation, walls and roof. And in these sections, you can see what we are going for.

House cross-sections, (north-south, at staircase).

We’re not deviating much from the standard way most Passive Houses do things with the foundation, which will be an engineered concrete slab with 12″ (c.305mm) of EPS insulation beneath and about 10″ (c.255mm) at the edges. The insulation continues out well beyond the foundation itself to provide a frost skirt around the whole foundation. This is the only significant amount of petroleum product we’ll be using in the whole house and one that couldn’t be avoided.

The external walls of the house are 110mm (c. 4 1/3″) Cross-Laminated Timber (CLT), wrapped in 280mm (c.11″) of rigid wood fibreboard insulation, a vapour-permeable moisture barrier, and local white cedar siding. The CLT sits on the concrete foundation and the fibreboard forms a continuous layer of insulation with the EPS beneath.

The roof is also CLT, but there’s a lot more fibreboard insulation up there: around 360mm (c.14 1/6″). The external roof will be steel. This will last a long time, and when it does eventually need to be replaced, it can be recycled.

So how will this design perform? What about R and U values? The wall structure should be just over R50 (under U0.02), and the foundation significantly better. The roof structure will be more than R60 (under U0.017). There will be other houses with higher claimed R-values and lower U-values, but the figures here are not claims but are based on tested values for the components, certified by the Passivhaus Institut in Darmstadt, Germany. What’s more, with the precision cutting of the manufacturing – we’re talking 1mm tolerances – the tightness of the house will be unparalleled, and the windows and doors (about which more in a later post) will be much better performing than any equivalents more commonly available in Canada. There are other considerations when it comes to what kind of R- or U-values we want to aim for. Firstly, we’re by no means in the coldest part of Canada: -25 Celsius is about as cold as it gets here in February – and it is never that cold for more than a few days at a time – compared to -45 in places like Edmonton. Secondly, we will have to burn some wood for water heating anyway, which means the house will never be anything less than comfortably warm in winter. And finally, we will already be exceeding Passive House standards. In short, pushing for much higher performance would mean significantly increased costs and a more bulky look to the house, and all for very limited noticeable gain; we think we’ve got the balance right.